The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and, more particularly, to an image forming apparatus having a toner recycling mechanism.
It has been customary with an image forming apparatus of the type described to remove toner left on a photoconductive element or image carrier after image transfer, temporarily store it in a cleaning device or similar toner collecting means, and then discard it. However, a current trend in the imaging art is toward the recycling of the used or waste toner for the protection of environment and ecology. To recycle the toner, a transfer belt may be used to remove paper dust and other impurities from the toner before the toner is brought to the cleaning device, as proposed in the past.
In a repeat copy mode, it is a common practice to maintain both a bias voltage for image transfer and a pretransfer lamp (PTL) in an ON state not only during the image transfer but also during the interval between consecutive sheets. Particularly, a bias voltage for image transfer is continuously applied even during the interval between consecutive sheets, i.e., when image transfer is not effected. As a result, toner particles deposited on the drum at the interval between sheets and charged to the polarity opposite to the expected polarity, e.g., negative polarity are directly conveyed to a cleaning device which cleans the drum. Consequently, the toner of opposite polarity is conveyed to a toner hopper via the cleaning device together with the usual toner remaining after image transfer. This part of the toner is apt to cohere in the hopper and to have its polarity disturbed in the hopper due to aging, resulting in defective images. In this respect, Japanese Utility Model Laid-Open Publication No. 2-121765 teaches an implementation for reducing the quantity of light to issue from a discharge lamp in the event of image transfer.
The transfer belt in movement carries thereon the toner scattered around without being transferred to the sheet, the toner directly deposited on the belt, and paper dust which comes from the sheet. Preferably, the toner and paper dust, i.e., impurities should be prevented from being returned to the developing device. Some different approaches have been proposed to remove the impurities from the belt by cleaning the belt, and then discarding them while returning the other toner or collected toner to the developing device. Japanese Utility Model Laid-Open Publication Nos. 58-29480, 58-79767, 58-29480, 60-41580, 56-14277 and 58-189674, for example, each discloses a toner recycling system which separates toner particles of opposite polarity by use of an electric field. However, all these schemes need exclusive bias applying means and result in a bulky and complicated apparatus.
There has also been proposed a color copier in which an electric field for image transfer is applied only when a sheet exists between a photoconductive drum and a transfer belt. However, this scheme is not directed toward the collection and reuse of toner left after image transfer.